Sintered ferrite magnets having a magnetoplumbite-type (M-type) crystal structure are employed as permanent magnets which are the core components of small type motors for automobiles, and motors for electric equipments/home appliances.
Conventional M-type Sr- or Ba-based sintered ferrite magnets are generally prepared by the following procedures; First, an iron oxide is mixed with a carbonate of Sr or Ba, and the mixture is subjected to a reaction to form SrO.nFe2O3 or BaO.nFe2O3 by calcination. The resulting calcined material is pulverized to obtain a coarse powder. The coarse powder is mixed with a sintering aid such as SiO2, SrCO3, and CaCO3, and then wet pulverized to obtain a slurry of a fine powder having a uniform particle size. For the purpose of enhancing the magnetic properties such as the intrinsic coercive force (iHc) of the final product, Cr2O3 or Al2O3 is added to the fine powder slurry together with the sintering aid. The resulting fine powder slurry is molded using a pressing apparatus under a magnetic field, dried, sintered and processed to prepare a magnet with a desired shape.
Recently, improved performance characteristics and higher efficiency are required of automobiles, electric apparatus, and home appliances. To meet such demand, there have recently been conducted many studies to develop a permanent magnet having improved magnetic properties in terms of the residual magnetic flux density (Br), intrinsic coercive force (iHc), squareness (Hk/iHc), and maximum energy product (B.Hmax).
For example, European Patent Laid-Open Publication No. 0905718 discloses a sintered magnet comprising a primary phase of a hexagonal ferrite containing A, R, Fe, and M, which is represented by A1-xRxFe12-yMyO19, wherein A is at least one element selected from the group consisting of strontium, barium, calcium, and lead, with strontium being essentially contained in A; R is at least one element selected from rare earth elements including lanthanum, bismuth, and yttrium, with lanthanum being essentially contained in R; M is cobalt or a mixture of cobalt and zinc (the proportion of cobalt in M is at least 10 atomic %); and the proportions of A, R, Fe and M elements with respect to the total amount of the metallic elements are in ranges of 3 to 9 atomic %, 0.5 to 4.0 atomic %, 86 to 93 atomic %, and 0.5 to 3.0 atomic %, respectively.
European Patent No. 0940823 B1 (corresponding to Japanese Patent No. 3,181,559 and U.S. Pat. No. 6,402,980) discloses a sintered magnet comprising a primary phase of a hexagonal ferrite containing Ca, R, Fe, and M, which is represented by Ca1-xRx(Fe12-yMy)zO19, wherein M is at least one element selected from the group consisting of Co, Ni, and Zn, with Co being essentially contained in M (the proportion of cobalt in M is at least 10 atomic %); R is at least one element selected from rare earth elements including La, Y, and Bi, with La being essentially contained in R; and the proportions of Ca, R, Fe and M elements with respect to the total amount of the metallic elements are in ranges of 1 to 13 atomic %, 0.05 to 10 atomic %, 80 to 95 atomic %, and 1.0 to 7.0 atomic %, respectively.
U.S. Pat. No. 6,139,766 discloses a sintered ferrite magnet having the composition of A1-xRx(Fe12-yMy)zO19, wherein A is at least one element selected from the group consisting of Sr, Ba, and Pb, with Sr being essentially contained in A; R is at least one element selected from rare earth elements including La and Y, with La being essentially contained in R; M is cobalt or a mixture of cobalt and zinc; and x, y and z satisfy the conditions of 0.04≤x≤0.9, 0.04≤y≤0.5, and 0.7≤z≤1.2, respectively.
Korean Patent No. 10-0839206 discloses a magnetic material comprising a primary phase of a hexagonal ferrite, the primary phase being represented by LaxCamα1-x-y(Fe12-yCoy)z with α being Ba, Sr or a mixture thereof, wherein the constituent ratios of the metal elements constituting the primary phase satisfy the following conditions: in the region bound by the points, A:(0.53, 0.27), B:(0.64, 0.27), C:(0.64, 0.35), D:(0.53, 0.45), E:(0.47, 0.45), and F:(0.47, 0.32) in the (x, m) coordinates, 1.3≤x/yz≤1.8, 9.5≤12z≤11.0, and (1−x−m)/(1−x)≤0.42.
Such conventional ferrite magnetic materials, however, still show unsatisfactory magnetic characteristics. Accordingly, efforts are continuing to develop magnetic materials having improved magnetic properties to meet the recent requirements, i.e., high performance, high efficiency, miniaturization, and weight reduction of motors for automobiles and motors for electric equipments as well as for home appliances.